'Man from Denmark'. 'Viking'
An experienced genealogist with the surname 'Dench', and some considerable research in this exact question.

The meaning of the word today, is still no different from that of its original use.

The Vikings that frequented the British Isles's past, were a far larger and stronger specimen, compared to those of the native inhabitants. Their fighting style, spirit and berserker rage in the face of overwhelming odds, made them men of renown, historically hired by Kings throughout Britain, to settle differences in times of war.

is similar to the way the Vikings were perceived with their feats of strength, battle and bravery, augmented with presence and rage.

In fact even the qouted

"When man gets punched in the face: You say "Thass denchhhh!!"

Could have been the sound of a commonly used alarm.
"The alarm 'DENCH!' (Men from Demark) is raised by the watchman as the first of the invading Vikings, sinks his axe into the head of the other sentry."

a blending of the word 'hench' and 'dark' used by a popular footballing icon. it refers to someone who is well built and this can also include the penis. for example 'my penis is well dench'. however it can also be used an adjective. it is generally a word used by male teenagers who are insecure and twattish.